This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In a mobile network such as LTE (Long Term Evolution), there are two main DL (downlink) transmissions on PDCCH (Physical Downlink Control Channel) (e.g. in 3GPP Rel-8/9/10) or on EPDCCH (Enhanced Physical Downlink Control Channel) (e.g. in 3GPP Rel-11):                UL (uplink) grant: a grant to a UE to transmit data on PUSCH (Physical Uplink Shared Channel), which is also referred to as PUSCH grant; and        DL assignment: an assignment to the UE that there are DL data available for the UE on PDSCH (Physical Downlink Shared Channel), which is also referred to as PDSCH assignment.        
In addition, there are also DL data related to the DL assignment transmitted on PDSCH, which are also referred to as PDSCH data, and could be real payload data to the UE, dummy data, etc.
UE-specific UL grant and DL assignment are, in a DL subframe, signaled independent to each other through PDCCH/EPDCCH.
Correspondingly, there are two main UL transmissions:                UL data: these are data sent by the UE on PUSCH in response to the UL grant, which are also referred to as PUSCH data; and        HARQ (Hybrid Automatic Repeat reQuest) ACK/NACK: this is a response sent by the UE to indicate whether the PDSCH data are received by the UE correctly or not.        
The HARQ ACK/NACK can be sent on PUCCH (which is also referred to as PUCCH HARQ ACK/NACK) or on PUSCH (which is also referred to as PUSCH HARQ ACK/NACK) where the HARQ ACK/NACK is multiplexed with the PUSCH data. This depends on e.g. whether the UE supports simultaneous PUSCH and PUCCH transmissions (which is a capability of UE that was introduced in 3GPP Rel-10, in which the HARQ ACK/NACK is transmitted on PUCCH only); or whether the PUSCH data are sent in the same UL subframe in a case that the UE does not support the simultaneous PUSCH and PUCCH transmissions; and also depends on whether a skipPadding bit is set for the UE (e.g. in 3GPP Rel-13), which will be discussed later.
In the case that the UE does not support the simultaneous PUSCH and PUCCH transmissions, the HARQ ACK/NACK will be sent on the PUCCH unless there is a coinciding PUSCH transmission in the same UL subframe. In a case that there is a coinciding PUSCH transmission in the same UL subframe, i.e., the UE has received a PUSCH grant (on PDCCH/EPDCCH) for the same UL subframe as for which the HARQ ACK/NACK will be transmitted, the HARQ ACK/NACK will be sent (i.e., multiplexed) together with the PUSCH data on PUSCH resources.
In the case that the UE is configured for supporting the simultaneous PUSCH and PUCCH transmissions, the HARQ ACK/NACK will be sent on the PUCCH.
In LTE, there is about 1% target block error rate for PDCCH/EPDCCH. For comparison, a BLER (Block Error Rate) for PDSCH and PUSCH is typically around 10 times higher, i.e. about 10%, and a probability for missed detection for HARQ ACK/NACK on PUCCH is around 1%. For robustness reasons, the access node, e.g., eNodeB, applies two separate energy detection schemes, i.e., DTX (Discontinuous Transmission) schemes, to find if an UL grant and a DL assignment have been received by the UE, respectively.
For the UL grant, the eNodeB tries to detect an UL signal at allocated PUSCH resources. If there is no UL signal received, e.g. no energy detected at the allocated PUSCH resources, the eNodeB will consider it as a DTX, which may be caused by either the UL grant being not received or erroneously decoded by the UE, or the PUSCH data being transmitted from the UE but not received by the eNodeB. The eNodeB will then schedule an UL grant re-transmission.
For the DL assignment, eNodeB tries to detect the HARQ ACK/NACK transmitted on UL, either on PUCCH or on PUSCH, to determine if the previous DL data are received by the UE. If there is no HARQ ACK/NACK detected on UL at the expected resources where the HARQ ACK/NACK shall transmit, the eNodeB will consider the DL assignment as either not detected or the UL HARQ ACK/NACK as not received. A DL re-transmission will then be scheduled by the eNodeB.
It should be noted that when the HARQ ACK/NACK is multiplexed with the PUSCH data, decoding results for the PUSCH data and the HARQ ACK/NACK multiplexed with the PUSCH data could be different, i.e. the PUSCH data could be correctly decoded while the HARQ ACK/NACK multiplexed with the PUSCH data is erroneously decoded. Since the BLER target for the PUSCH data on PUSCH is around 10% and for the HARQ ACK/NACK multiplexed with the PUSCH data is around 1%, it is actually the reversed results that are more likely, e.g. PUSCH data are erroneously received and the HARQ ACK/NACK multiplexed with the PUSCH data is correctly decoded.
For the DL assignment case, there are two types of errors that could cause DTX:
1) the PDCCH/EPDCCH is not received correctly or erroneously decoded by the UE, or
2a) the HARQ ACK/NACK from the UE to the eNodeB is lost for some reason.
For the UL grant case, there are similarly two types of errors causing DTX:
1) the PDCCH/EPDCCH is not received correctly or erroneously decoded by the UE, or
2b) the PUSCH data from the UE to the eNodeB is lost for some reason.
For eNodeB, it is very important to distinguish between these two DTX cases, e.g. with respect to Link Adaptation, since the first DTX case 1) belongs to a DL problem, while the second DTX cases 2a) and 2b) belong to an UL problem.
As previously described, in 3GPP Rel-13, a skip-padding-in-uplink alternative is introduced (which is disclosed in e.g. R2-156296, R2-157032, R2-157033 available from http://www.3gpp.org/). When the “skipPadding” bit is set and signaled to the UE (which is also referred to as “skipPadding UE”) through a RRC signaling or on a separate dynamic grant, the UE is allowed to not transmit at all if there is no data in an UL buffer. However, the UE in previous 3GPP releases (which is also referred to as “legacy UE”, in order to be distinguished from the skipPadding UE in e.g. 3GPP Rel-13) was obliged to transmit on granted PUSCH resources even when it had not enough data in the UL buffer of the UE; and then the UE transmitted padding data. Hereinafter, “UE” is used for referring to either the legacy UE or the skipPadding UE, unless otherwise indicated.
Thus, when such a new skipPadding option is used, the eNodeB cannot know if the DTX on PUSCH is due to a third DTX case:
3) no data in a buffer of the skipPadding UE (“true DTX”),
or if the DTX is due to one of the previous two types of DTX cases. It should be noted that this third case 3), i.e. no data in the UE buffer, is not an error case, but rather a “true DTX” case.
This third DTX case is only applicable to the UL grant.
There may be problems with the existing technical solutions in which the UL grant and the DL assignment are signaled independent to each other on PDCCH/EPDCCH.
For example, when the eNodeB transmits only the UL grant or only the DL assignment to the UE on PDCCH/EPDCCH and gets no feedback from the UE, i.e., detects a DTX, it is impossible for the eNodeB to determine whether the DTX is caused by a DL problem (e.g. UL grant or DL assignment on PDCCH/EPDCCH being erroneously decoded or being not received by the UE) or by an UL problem (e.g. PUSCH data or HARQ ACK/NACK being lost), which disadvantageously affects DTX robustness.
Especially for the skipPadding UE, there is an additional DTX case that cannot be differentiated by the eNodeB from the two DTX cases for legacy implementations. This causes problems for the eNodeB e.g. w.r.t. link adaptation when it is not known if a PDSCH was transmitted or not.
Furthermore, with independent UL grant and DL assignment transmissions on PDCCH/EPDCCH, the UE might require at least two separate PDCCH/EPDCCH physical resources, at least one for UL and at least one for DL scheduling, simultaneously. Thus, there is a waste of the PDCCH/EPDCCH resources, which causes PDCCH/EPDCCH resource utilization inefficiency. This is extremely important in NX (Next generation), where simultaneous UL and DL transmissions in the same subframe is more frequent.
Therefore, a technical solution of enabling the eNodeB to differentiate whether a DTX is caused by a DL problem (UL grant or DL assignment on PDCCH/EPDCCH being erroneously decoded or being not received by the UE) or by an UL problem (e.g. PUSCH data or HARQ ACK/NACK being lost) is desired so as to improve DTX robustness.